//计算服务器消耗时长的中间件
module.exports = async (ctx,next) => {
	//记录开始的时间
	const start = Date.now()
	//让内层中间件得到执行
	await next()
	//记录结束时间
	const end = Date.now()
	//设置响应头X-Response-Time
	const duration = end - start
	//设置响应头
	ctx.set('X-Response-Time',duration+'ms')
}